1. Field of the Invention
The present invention relates to a magnetic position detector including a scale section made of a magnetic material and a sensor section provided independently from this scale.
2. Description of the Related Art
Conventionally, a position detector in which a scale section and a sensor section are separated has been used for position detection of, for example, a main shaft of a machine tool. In such a position detector, the scale section is made of a magnetic material, and the sensor section has a magnetic position sensor on its surface close to a circumference surface of the scale section. Because such a position detector uses magnetic field for position detection, it has excellent environmental resistance to water, oil, and the like. In addition, scale sections of different sizes can be manufactured without difficulty by machining. There is a further advantage that it is possible to use a common sensor section for the differently sized scale sections.
FIG. 4 shows a top view of the above-described conventional position detector. A scale section 2 is composed of, for example, a cylindrical soft magnetic material and fixed to a rotation shaft 1 on the detection target side. The scale section 2 also has concavities and convexities of a gear pitch λ on the circumference surface. A sensor section 4 is fixed on an installation surface 9 on the non-rotational side of a detection target using mounting bolts 7 and 8, and a magnetic position sensor 5 is located on a surface 6 close to the circumference surface on the scale section 2 side. When the rotation shaft 1 rotates, the magnetic position sensor 5 detects a magnetic change caused by the concavities and convexities on the circumference surface of the scale section 2. The sensor section 4 detects a rotational position of the rotation shaft 1 based on the magnetic change detected by the position sensor 5 using an incorporated signal processing circuit. The sensor section 4 further transmits the detected rotational position information to an external device via a cable 11 using serial communication.
With the position detector as shown in FIG. 4, there is a problem that detection accuracy of the position detector deteriorates when a contacting circumference surface 3 of the scale section 2 and a surface 6 on which the position sensor 5 is positioned are not positioned in parallel to each other with sufficient accuracy, that is, when the interval between them is not uniform. For this reason, in the conventional example of FIG. 4, a guide surface 10 is provided on the installation surface 9 on the non-rotational side of the detection target. This guide surface 10 serves to fix the scale section 2 to the installation surface 9 such that the contacting circumference surface 3 and the surface 6 of the sensor section 4 are parallel to each other with a uniform interval in between. Additionally, as a method of fixing a sensor section, as in Japanese Laid-Open Publication No. Hei 1-239412, a method has been widely known in which a fixing jig is temporarily inserted between a scale section and a sensor section to fix the sensor. Position detectors are described in, for example, Japanese Laid-Open Publication No. Hei 1-239412, Japanese Laid-Open Publication No. Sho 63-205514, and Japanese Laid-Open Publication No. Hei 5-87512.
With the conventional position detector shown in FIG. 4, it is necessary to accurately machine guide surfaces for fixation use on both measurement target side and sensor section side. Providing such an accurate guide surface entails difficult machining and assembly, resulting in cost increase. Further, in a method using a fixing jig, when differently sized scale sections are manufactured, a dedicated fixing jig is necessary for each size. A wide variety of fixing jigs are consequently necessary to be provided for replacement in case of failure, etc. Even when a fixing jig is used, an accurate guide surface is still necessary in a position in contact with a fixing jig on the sensor section side.